He was compulsory reading for many schoolboys in the 1950s. Caught between the heel of the Nazi jackboot and the tide of the red menace, the novels were paranoid, mythical and racy, mixing the occult, anti-revolutionary politics and espionage. They were terribly literal. Dennis Wheatley believed communism was the work of the devil. At the same time he didn't take himself too seriously as a writer and had an endearing habit of alerting the reader to boring chunks of historical and political analysis that could be skipped.

He sold more than 50 million books - his best-known is The Devil Rides Out (1934), made into a film by Hammer in 1968 - but he is now eclipsed and remembered, if at all, as a poor man's Graham Greene. Greene was, famously, author and agent, including a spell at the Ryder Street "school for spies" with Kim Philby. Wheatley's clandestine war, the subject of this book, was less romantic and barely urgent. He spent six weeks prior to September 1939 holidaying in France, ignorant of political developments. Turned down by the Ministry of Information, he eventually fetched up at the War Office, thanks to his wife, who was a driver for MI5, where he was hired as an established author to dream up scenarios that read like drafts of his fiction, speculating on the likelihood of a German invasion and how to combat it.

Later he moved on to deception strategies, including the idea of creating a rumour of the arrival of a pacifist Messiah in Nazi Germany. Though that idea was rejected, he worked on plans that were adopted, including a phantom invasion of Norway and the use of Montgomery's double. Generally, the British government got a better return from his analyses than Himmler did from astrology.

Given the current neglect of Wheatley, limiting this book to his government papers does him little service. He wrote as an extrovert, in fast, misspelled English, producing 7,000 words a day; one 15,000-word paper involved 200 cigarettes and three magnums of champagne. His paper on the resistance to German invasion offered a grand, paranoid and often impractical vision ("All houses in Zone 3, except those being used for defensive purposes, should have a boobytrap laid in them"), characterised by fifth-form prose and ingenuity. He suggested dropping leaflets on German troops that said: "Come by air and meet our new death ray (this sort of lie is good tactics at a time like this). Every Nazi visitor guaranteed death or an ugly wound. England is Hell - it's going to be just the same for you in either."

Wheatley's war remained unhindered by liberal thinking or rationing. He lunched at Rules (a favourite of Greene's), starting with two or three Pimms at the bar, followed by a "short one" well laced with "Chanel 5", as a colleague called absinthe, wine with the meal (specials included jugged hare and game; Welsh rarebit to follow) and ending with port or kummel, followed by an office siesta, with instructions to be woken with a solitary ring on the telephone if urgent. For all the hysteria of imminent invasion, the reality was less the paranoia of Wheatley's papers than the knockabout of Dad's Army, with plenty of slacking (an hour of office time to spend on the Times crossword), swanking about with a blue-leather swagger stick and scarlet-lined greatcoat, and the booze; a war in fact fuelled by alcohol.

&middot Chris Petit's new thriller, The Passenger, will be published by Simon & Schuster in April.